Hemispheric Lateralisation

Hemispheric Lateralisation and Split-Brain Research

Hemispheric lateralisation refers to the functional specialisation of the two cerebral hemispheres — the left and right halves of the brain — for different types of processing. Whilst the two hemispheres appear anatomically similar and are generally capable of processing the same types of information, they show systematic tendencies towards different modes of processing, with many functions more strongly represented in one hemisphere than the other. The two hemispheres communicate primarily through the corpus callosum, a large band of myelinated nerve fibres connecting the left and right cortices.

Key Features of Lateralisation

The most well-established example of lateralisation concerns language: in approximately 95% of right-handed individuals and 70% of left-handed individuals, language production and comprehension are predominantly left-lateralised — controlled primarily by the left hemisphere. The left hemisphere is generally characterised as specialising in: analytical processing, logic, sequential reasoning, language (especially in right-handers), and fine motor control of the right hand. The right hemisphere is generally characterised as specialising in: spatial processing, holistic perception, facial recognition, emotional processing, music, and artistic ability.

Each hemisphere controls the contralateral side of the body — the left hemisphere controls voluntary movement and receives sensory input from the right side of the body; the right hemisphere controls and receives input from the left side. Visual input is similarly crossed — images in the left visual field are processed by the right hemisphere and vice versa.

Split-Brain Research

The most compelling evidence for hemispheric lateralisation comes from split-brain studies conducted by Roger Sperry and Michael Gazzaniga in the 1960s. Their participants were patients who had undergone corpus callosotomy — surgical severing of the corpus callosum — as a treatment for severe, intractable epilepsy (to prevent seizures from spreading between hemispheres). Following the surgery, the two hemispheres could no longer communicate directly and could be studied independently.

Sperry and Gazzaniga used a procedure in which participants fixated on a central point whilst stimuli were flashed briefly to the left or right visual field, ensuring the information reached only one hemisphere. Key findings included:

• Objects shown to the right visual field (left hemisphere): participants could name the object verbally — confirming left hemisphere language dominance.
• Objects shown to the left visual field (right hemisphere): participants could NOT name the object but could select it correctly with their left hand from a collection of objects — demonstrating that the right hemisphere understood the object but could not produce language to name it.
• The two hemispheres appeared to process information independently and sometimes produced conflicting responses — the left hand (right hemisphere) was 'unaware' of what the right hand (left hemisphere) was doing, and vice versa.
• The left hemisphere, when asked to explain right hemisphere actions it was unaware of, confabulated — producing plausible but false explanations, suggesting an active narrative-construction function of the left hemisphere.

Sperry was awarded the Nobel Prize in Physiology or Medicine in 1981 for this work.

Evaluation

Strengths: split-brain studies provided the most definitive evidence for hemispheric specialisation, demonstrating that the two hemispheres process information differently and independently when their communication is severed. The findings have been enormously influential in neuroscience and psychology. Limitations: the participants were a highly unusual population — individuals with severe epilepsy who had undergone radical surgery. Their brains may have been atypical before surgery, and the epilepsy itself (and its surgical treatment) may have altered brain organisation. Generalising split-brain findings to neurologically intact individuals requires caution. Additionally, modern research has shown that even in split-brain patients, the two hemispheres coordinate their activity more than early research suggested — subcortical pathways remain intact and provide some interhemispheric communication even after corpus callosotomy.